When lemming numbers were high, the stored eggs made up less than 28 percent of the foxes' springtime diet.

When the rodents were scarce, the eggs accounted for up to 74 percent of the mammals' food.

For the foxes, the eggs are a reliable backup system because they are abundant during goose nesting season and are well suited to long-term storage.

"Eggs are protected by the egg shell, several membranes, as well as chemical properties of the albumen [egg white], preventing microbial activity," Samelius said.

The cold conditions of the Canadian Arctic also extend the shelf life of stored eggs, according to the study team.

Chemical Proof

The new study monitored the arctic foxes' yearly diets using analysis of chemical markers called isotopes in samples of the animals' fur and blood.

Different levels of carbon and nitrogen isotopes in these samples reflected the different foods the mammals ate in the spring and fall of a given year.

Vincent Careau is an arctic-fox researcher at the Université de Sherbrooke in Quebec who was not involved with the study.

He said that the new research offers solid evidence for what many biologists had previously suspected.

"Many noted the presence of egg shells in fox scats while there was no goose around, meaning that the consumed egg was inevitably recovered from a cache," Careau said, "but these observations remained anecdotic until this study came out."

How the foxes manage to locate their egg caches after such a long time remains a mystery, Careau added.

But studies of closely related red foxes and observations of the arctic foxes suggest that the animals use "spatial memory of cache locations and exploratory digging," he said.

"We also observed foxes spending a lot of time moving eggs from cache to cache," Careau added.

"This re-caching behavior probably improves memory of cache locations, especially if they progressively bring their eggs into one basket."